If you have access to a drill press do this. Ream or drill the manifold very careful so it stays flat for a good straight hole and only this hole. Then mount the manifold "perfectly in place" with the other two studs. Now tap by getting the tap started in an existing thread and tap "dry" no oil or lube if you wish to use the JB. The quality of the threads will depend on how thick the block still is there.
If there is little quality thread perhaps going to a 7/16" helicoil may do you well.
Dave Neptune